The laser ignition of fuel droplets, such as those found in the combustion chamber of a gas turbine engine, provides several significant advantages over conventional spark plug type ignitors. For example, with a laser ignitor the combustion process can be started in a more optimum chamber position. Spark ignitors are typically positioned at a peripheral, non-optimal position of a combustion chamber, while the fuel spray to be ignited is located in a central portion adjacent to a fuel injector. Also, a laser ignition system that operates outside the combustion chamber, with an appropriate optical coupling into the fuel spray, is not subject to the type of degradation experienced by internally mounted spark ignitors. This degradation of spark ignitors is known to cause eventual failure after prolonged use.
In U.S. Pat. No. 4,947,640, issued Aug. 14, 1990, entitled "Gas Turbine Engine Photon Ignition System", two of the present inventors describe method and apparatus for igniting a hydrocarbon fuel that is comprised of droplets of hydrocarbon fuel. The hydrocarbon fuel is provided as an air/fuel spray. Electromagnetic radiation having wavelengths primarily within a range of approximately 185 nm to approximately 400 nm (UV) is generated and directed into the air/fuel spray. The droplets absorb the energy, are heated, fragmented and ignited. The use of electromagnetic radiation within the ultraviolet region is shown to be beneficial because of a high absorption of radiation within this wavelength range by hydrocarbon fuels such as JP-4 and JP-5.
The following U.S. and foreign patents are cited as relating to the ignition of and/or the preconditioning of fuels with electromagnetic energy:
U.S. Pat. No. 4,035,131, issued Jul. 12, 1977, entitled "Control of the Initiation of Combustion and Control of Combustion" by A. E. Cerkanowicz; U.S. Pat. No. 4,726,336, issued Feb. 23, 1988, entitled "UV Irradiation Apparatus and Method for Fuel Pretreatment Enabling Hypergolic Combustion" by L. O. Hoppie et al.; U.S. Pat. No. 3,258,910, issued Jun. 8, 1962, entitled "Fiber Optic Ignition" by R. J. Seymour; U.S. Pat. No. 3,473,879, issued Oct. 21, 1969, entitled "Shock Wave Burner" by B. Berberich; U.S. Pat. No. 3,861,371, issued Jan. 21, 1975 entitled "Ignition System for Engine" to J. Gamell; U.S. Pat. No. 4,416,226, issued Nov. 22, 1983, entitled "Laser Ignition Apparatus for an Internal Combustion Engine" by M. Nishida et al.; U.S. Pat. No. 4,434,753, issued Mar. 6, 1984, entitled "Ignition Apparatus for Internal Combustion Engine" to Mukainakano et al.; and two U.K. Patents to D. Brown, both entitled "Ignition Systems" specifically: 1,236,561, published Jun. 23, 1971, and 1,360,196 published, Jul. 17, 1974.
The following two technical reports are cited for teaching the ignition of premixed flowing gases with electromagnetic energy: Brad E. Forch et al., Technical Report BRL-TR-27409, U.S. Army Ballistic Research Laboratory, entitled "Photochemical Ignition Studies. II Oxygen-Atom Two-Photon Resonance Effects", June, 1986; and Andrezik W. Miziolek et al. Technical Report BRL-TR-2644, U.S. Army Ballistic Research Laboratory, "Photochemical Ignition Studies. I. Laser Ignition of Flowing Premixed Gases", February 1985.
Finally, in U.S. Pat. No. 4,302,933, issued Dec. 1, 1981, entitled "Jet Engine Augmentor Operation at High Altitudes" by Marvin M. Smith there is disclosed a pulsed CO.sub.2 TEA laser that is employed to generate laser-supported absorption (LSA) waves within a jet engine augmentor. The LSA waves are initiated by a laser beam reflecting off of targets, such as fuel droplets, in the eye of a cyclonic air action in a recirculation zone of the augmentor. This is said to cause the emission of electrons which serve as priming electrons to break down air into a plasma of high temperature (10,000.degree. to 20,000.degree. K.). The LSA waves are said to be initiated by directing a 10.6 micron wavelength converging beam having a minimum intensity of 6.times.10.sup.8 W/cm.sup.2. The air breakdown is said to proceed via inverse bremsstrahlung heating. It is stated that a/hot air plasma of 1-2 ev is formed which propagates back up the laser beam away from the formed plasma where most of the laser beam energy is absorbed. The system of Smith is said to measure the ambient air pressure and to vary the pulse repetition rate of the laser such that the fuel-air ratio in the wave combustion is continuously maintained at a high thermal efficiency. Smith also makes reference to the use of "additives" that may be used in conjunction with the fuel.
The teaching of Smith does not address the problem of low powered and light-weight laser ignition systems for use within a gas turbine engine combustor. As is evident from his disclosure, Smith envisions a high powered laser source to generate LSA waves within the engine augmentor.
However, one important criteria for an aircraft laser ignition system is that the system be a low powered system having a small physical size and weight. It is thus one object of this invention to provide for a laser initiated ignition of a fuel spray within a combustion chamber with a low powered laser source.
It is a further object of this invention to provide for a laser initiated non-linear ignition process of a fuel spray within a combustion chamber of a gas turbine engine.